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Introduction: Guitar effects controllers and MIDI

As a longtime guitarist I have watched and participated in the technological changes around guitar effects and how they are used by guitar players of all genres. My first commercial guitar effect was a phase shifter soon followed by a compressor and a distortion pedal. At some point I had mounted them all to a piece of scrap plywood painted black. That didn’t last long as my musical repertoire expanded I added an analog delay, a wah pedal and a chorus. I scavenged an old mixer case that had handles and was covered in tolex to house my pedals. There were no standards and many pedals ran on batteries only.

It did not take long before I was modifying pedals with LED indicators and power connectors. Soon I had other guitarists and friends asking for these modifications and I started building power supplies to eliminate batteries from these first generation pedal boards. Many high profile guitarists had custom pedalboard switching systems built by designers like Pete Cornish that were notoriously expensive. These designs were built for reliability and low noise required by the touring guitarist and were simply not available to the average guitar player.

I was dimly aware when MIDI was introduced in 1983 but as a guitarist I thought it was only for keyboards and synths. By the mid 80’s manufacturers starting offering MIDI controlled effects systems for guitarists like the Ibanez EPP-400 and the Peavey MFP 2128. These early MIDI controlled effects systems were soon forgotten with the advent of digital and rack based processors. Designers like Bob Bradshaw rose to prominence as integrators of these rack based systems for high profile guitarists. Again the average guitarist could only dream about owning one of these rigs along with the power and flexibility they offered to control effects.

Fast forward to 2016 where things have come full circle and pedalboards are the norm for most guitarists. The last 5 to 10 years has seen a proliferation of effects pedals from mainstream manufacturers, boutique one man shops and a flood of low cost copies from Asia. Digital based pedals now rival effects once only available as rack mounted units designed for studio use. Even the beginner guitarist can now afford a pedal collection to create different tones and musical genres. As most guitarists will acknowledge these modern pedalboards require some kind of signal routing and management to effectively use the sonic possibilities during a performance. Fortunately technology marches on and the ability to control and manage effects is available to all guitar (and bass) players without the high price tag of a Cornish or Bradshaw designed rig.

My goal in writing these articles is to explain the technology around guitar effects systems and how MIDI can play a role in helping the guitarist streamline the use of effects during a performance. More and more manufacturers are offering effects pedals with MIDI capabilities and to realize the full musical potential of these devices requires an understanding of this technology and how it can be used in the creative process.


When the MXR Distortion III was first released I thought, great- MXR has finally decided to reissue/update the overlooked original Distortion II !  Alas the only thing close to the Distortion II was the name. The  pedal is a newer design that has more in common with other Tube Screamer type pedals that it sounds like. The design is a two stage op amp circuit with diode clipping to ground. The highs and lows are rolled off to give that signature 750Hz midrange boost found in the original TS design. It’s not a bad sounding pedal but its not distinctive. MXR later released the Classic Distortion pedal in conjunction with Guitar Center in the US. It was a Guitar Center exclusive and sold for $29.99 at their seasonal sales events. Eventually pedal enthusiasts on the internet noticed the two pedals sounded very similar. Somebody decided to investigate and discovered the circuits used are identical. The Classic Distortion has a lighter, thinner case and a different color but make no mistake these pedals use the exact same printed circuit board and component values.

Here is the schematic for the Distortion III. A popular mod to open up the pedals sound and raise the 750HZ low pass filter is to remove C4 (.022uf) from the circuit entirely or replace it with a smaller value like a .005uf capacitor. I choose a slightly different way to achieve similar results.

  • Unsolder and lift one end of C4 and connect a 2.4K resistor in series with C4 to the open solder pad where C4 had been connected. If you don’t have a 2K4 resistor try a 2.2K or a 2.7K.
  •  Solder a .0022uf film cap across R5  on the solder side of the PCB.
  • Remove the clipping diodes D1 and D2. Replace these with two 2N7000 MOSFETs with the gate tied to the drain. Place a 1N34 diode in series with one MOSFET to raise the clipping threshold on one side for a little asymmetry.
  • To be clear the MOSFETs are connected so the intrinsic body diode is clipping the signal and not the MOSFET junction itself.
  • Replace the stock LED with a nice <insert your favorite color>one.

The pedal now has a nice upper mid-range push that can be tamed with the tone control if desired. The MOSFET /germanium combination gives an open more articulated sound to chords and single notes. I think the softer clipping of the MOSFETs is complemented by the extended high end. The overall result is very good and there is an apparent increase in the overall distortion level when compared to the stock pedal.

Recently Digitech has released a new version of the venerable FX 10 BiFet preamp, The FX10 was an updated design of the original DOD BiFet 410 Studio Preamp. The original 410 was designed as a preamp for acoustic guitars with piezo pickups. It has a very high input impedance with no foot switch so it was an always on device.


There is also an XLR ouput connector next to the standard output jack presumably to go direct to a mixer for SR or recording. The “BiFet” label is sometimes misinterpreted as there two or “bi” FET devices in the circuit. BiFET actually refers the type of op-amp used with an FET input and a bipolar transistor output. The combination of these two device types produces an op-amp with good high impedance input with the FET  and an improved bipolar output with low impedance and good current drive abilities. The tone control is a “tilt” type EQ that provides simultaneous treble boost/bass cut or bass boost/treble cut. It is a very effective type EQ  that gives subtle shifts in tone around the mid point from 10 to 2 o’clock on the knob. The same basic tilt EQ is used in quite a few DOD overdrive and distortion pedals.


Here is the schematic for the original 410 Studio Pre-amp. It is a simple single op-amp circuit easily built by a diyer.  I have included  a circuit snippet from the FX10 schematic for comparison. The FX10 is a more complicated circuit with FET switches incorporated. You could build it on a piece of perfboard, or if you have the abilities I have included a simple PCB layout below.



This is a great circuit for an always on buffer or to follow an over drive or distortion that needs some post EQ. Try this circuit after a standard cry baby wah to see how it can improve the sweep.

I have been trying out RAT type pedals recently and when I came across this one on ebay. I bought it immediately.

Vick Audio is a relatively new pedal builder out of Arizona. The 85 Classic Rat pedals “use the original LM308N chips that were considered the “secret” to these vintage Rat’s amazing tone.”

As soon as the pedal arrived I plugged it in and was greeted by an extreme, almost fuzz, tone from the pedal even with the gain set very low. It did not sound like an original Rat at all. I thought maybe there was something wrong with the pedal and that’s why the price was so low (retail price is $109). I opened it up as I do with all new pedals and started trouble shooting.


The circuit does indeed use the LM308N op amp with what appear,at first glance, to be the original component values. The PCB was “floating” inside the case only being supported by the leads attached to the board. There is a thin layer of orange foam on the trace side of the board to prevent shorts if it flops around inside the case.


If you look closely at the side view of the PCB above you can see there are four electrolytic capacitors ( the black cylinders with white lettering). There are two 100uf 16 v capcitors and two 4.7 uf capacitors on the board. One of the 100uf caps is used as a filter for the power input but the second is out of place in a RAT circuit. The traditional RAT circuit uses a pair of series resistor/capacitor circuits to set the high pass gain of the op amp. One pair is a 560 ohm resistor and a 4.7uf capacitor with a 60Hz roll off and second pair with a 47 ohm resistor and a 2.2uf capacitor with a 1.5kHz roll off. More technical analysis of the RAT circuit is available here. In essence the Vick Audio circuit had a 100uf capacitor in place of the more traditional 2.2uf.  This extended the full gain of the circuit down to 31Hz, well below any standard guitar fundamentals. This should create a fuzzy, flabby sound which is exactly what it sounded like.

I searched my parts bins for a 2.2uf cap but the smallest electrolytic I had was a 4.7uf so  I removed the 100uf and replaced it with the 4.7uf. Instant RAT tone! The 4.7uf does give a much lower roll off (677Hz) than the stock value but it still sounds quite good at lower gain settings. Once you wind the gain up past noon it does give  a fuzz type quality to the sound. I think most RAT users find the best sounds below 12 o’clock on the gain knob. One other change I made before closing up the pedal was to add a 150 ohm resistor in series with the gain control. This helps smooth out the first little bit of gain jumpiness when you start to turn the gain knob from the fully counter-clockwise position.

One last thing. The PCB is silk screened with the words “Rat Tail”. Vick audio does not sell a pedal called Rat Tail but these guys do: Wonder if there is any connection?


Here are some internals of the T-Rex Yellow Drive. The Yellow Drive is produced for Guitar Center in the US exclusively by T-Rex. The circuit is a copy of a Proco RAT circuit with some minor variations affecting the low end roll off and available gain. No surprise that the T-Rex Mudhoney and Mudhoney 2 are the same basic RAT type circuits. It also shows how a classic circuit can be marketed as a “different” pedal with only minor changes.

The A side of the Yellow Drive uses a 4k7 gain pot and a low end roll off @720Hz. The B side has 47k gain pot and a roll off @1kHz.  A useful combination to dial in a medium gain on A and a high gain sound on the B side. One other change that T-Rex implemented in  their variations on the RAT circuit is add a 150 ohm resistor in series with the gain pots to set the minimum gain to a usable level.

I don’t own an original RAT to compare but I think the Yellow Drive is a good option to a new production RAT considered it’s flexibility with two circuits in one box and the ability to fine tune the A channel for low to medium gain sounds.

I posted this over at the GAB (Guitar Amp Board) forum earlier this year.  I thought I would re-post it here with some updates and links to sites about biasing tube amps.

WARNING!  Before you attempt to modify any tube amp be sure you KNOW, UNDERSTAND and FOLLOW all safety precautions for working with high voltages. Failure to follow safety procedures may result personal injury and/or death by electrcution. It is recommened to have a qualified technician perform the following modification procedures.

Here is a mod for the Jet City JCA50 to allow external bias test points and adjustment per tube. You can then replace tubes and set the bias without removing the chassis from the head. The goals of this mod are safety and convenience. By using the cathode sense resistors to monitor the current through the tubes you eliminate the shock hazards trying to monitor plate current with an open chassis. I encourage everyone to avoid trying to bias your amp using the transformer shunt method shown in some You Tube videos. The transformer shunt method of measuring plate current is inherently unsafe. Buy a bias probe or at least install the cathode sense resistors on your amp.

The individual bias pots allow you to balance the DC currents through the two primary sides of the output transformer (OPT). Correct DC balance of the transformer primary reduces hum and provides better bass response by reducing any standing flux in the transformer primary. Matched tube sets can help when they are matched for current draw but there still may be a difference in the “matched” tubes. All tubes drift and matched sets are no exception. Check with your tube vendor to see if they burn in their power tubes before matching. Tubes may drift from their initial current draw over the first 10 hours of use. When installing new tubes, set the bias and let the amp idle for 4 to 5 hours then recheck the bias. The tubes will continue to drift as they age but less than the initial burn in period.

The Last Word on Biasing


The parts for this mod can be ordered through Weber VST.

2 ea PNJ-P-Red  Pin Jack

1 ea PNJ-P-Blk   Pin Jack

2 ea wp503L      50k linear pot,screwdriver slot with locking bushing

2 ea  1 ohm 1% metal film resistor

1 ea  15k ohm 5% 1/2 watt resistor

1 ea  10k ohm 5% 1/2 watt resistor

1 ea   16uF 150VDC electrolytic capacitor


First install the 1 ohm cathode sense resistors. This will allow safe monitoring of cathode current through the test points.

The JCA50 and 100 both have zero ohm jumpers on the circuit board that connect pin 8 (cathode) of the output tubes to ground.


These jumpers JP3 and JP2 are removed and the sense resistors installed in their place.

Here are the sense resistors installed


I have left the resistors above the board slightly to allow attachment of the leads and also to protect the board if one of the resistors gets taken out by excessive current. I am using 10 ohm 1/4 watt resistors. The 10 ohm allow better accuracy with most budget multimeters. The 1/4 watt rating is ok for my uses as it offers a margin of protection from a shorted tube taking out the OPT. You can use a 1 watt part here if you want. Be sure to get 1% resistors if you use the 1 ohm value.

Select a place to install the pin jacks. I installed these in a vertical line where they would not interfere with the removal of the PCB. Here is a shot inside the chassis. You can see how the test jacks clear the PC board on the left.


Select a location for the bias adjustment pots. I placed mine on the top of the chassis where they cannot be moved without taking the back panel off. Also I wanted the leads to be relatively short and to not use space on the back of the chassis. Note how the bias pots only have two wires attached to create a variable resistor.


One of the new bias pots will take the place of the original on board bias trimmer. Remove the trimmer from the board and connect the new bias pot in its place. This will be the bias adjustment for V8.


Unsolder and lift one end of R13 as shown. This will be where the new bias circuit will attach to bias V7.


For the new bias circuit you will need to construct some way to hold the additional parts. I choose a small piece of perfboard to hold the components. It is attached to the PC board with double sided foam tape. The new circuit is a replica of the one already on the board. Refer to the JCA50 power amp schematic. The new circuit consists of a 15k resistor, a 16uf 100v capacitor and a 10k resistor. The new bias pot will connect to this circuit and also the end of R13 that was lifted in the previous step. The circuit we are copying consists of R11, C6 and R18. VR1 is replaced by the new bias pot. Here is the schematic for the new bias circuit:


Remove all tubes from the amp if you have not already done so. Leave the standby switch OFF.

Power the amp on.

check between ground and pin 5 on V7 tube with a voltmeter

You should see a negative voltage between 63 VDC and 30 VDC

Rotate the V7 bias pot while reading the voltage. Make sure it is 60VDC or higher with the pot fully clockwise

Repeat testing for V8 bias

If testing is successful attach the leads from the test points to the cathode sense resistors. The black test point connects to ground and the red test points connect to the cathodes of V7 and V8 as shown here.


Using the new test points


Use your normal bias procedure for new tubes.
Measure your B+ voltage(you only have to do this once) and calculate the bias current for your tubes (if you dont know how to do this STOP now)
With the power tubes out check pin 5 for -DCV turn bias pots fully clockwise.
Power off-Insert power tubes- power on
Use a voltmeter connected to the test points
the voltmeter will display millivolts if you used a 1 ohm cathode sense resistor.
35mv = 35ma current
If you used a 10 ohm resistor add a zero
350mv = 35ma current

Here is an interesting device from the early eighties. Its a passive bass boost which connects inline between your guitar and amp.


No batteries, no switches, no knobs…but it works! This thing has been sitting around in a guitar case for years. I never really used it for guitar but I remember my bass player (DO) loved using this thing. It is completely sealed and potted so there is no way to open it up without destroying it. I am pretty sure it uses a small step-up transformer to achieve a small increase in level along with some treble cut to produce the effect. Zeus Audio also made some other devices you can find around but they are pretty rare. There was a straight Booster like this one and they also made a small practice  amp and a preamp/distortion device both of which were active and used a 9V battery.

The Radial Bigshot EFX is a 2 loop true bypass switcher. Each loop has its own in/out switch along with ground lift switches for each loop and a mute function that makes loop 2 a mute on/off. The enclosure is a nice heavy duty powder coated steel that has some heft to it. The only thing missing are indicators for the loop status.
The EFX comes with DPDT type switches directly attached to the PCB. The easiest way to install loop indicators is to replace the stock switches with two 3PDT type with the third pole switching the LED indicator on/off.
First remove the original switches from the PCB.

Next install the LED’s and the power connector.

Wire up the LED’s along with the limiting resistors to the switches.

Wire the new switches to the PCB pads of the original switches. The 3PDT foot switches should be mounted so the switch terminals clear the surface of the PCB. I threaded the internal switch nut all the way down against the switch body and the switches clear the PCB with no problems.

Test and close it up. Here is the finished mod. I choose an orange and a blue/green LED for the indicators.

One of the most frequently replaced parts on Boss pedals is the rubber grommet that the pedal thumbscrew rides in.
Many older pedals have this part completely missing or worn so badly the thumb screw is bent or started to wear the metal groove the screw rides in. A worn or missing grommet can cause switching issues not to mention the annoying clanking noise from the metal on metal contact.DSCN0252
You can order a replacement part from Boss for about $5 USD. Or you may find an ebay vendor selling replacements for inflated prices.
I use standard 7/16 or ½ inch diameter grommets available at most hardware stores and home centers, like Lowe’s or Home Depot. DSCN0253
You can also find these sizes in assortments from Radio Shack.

Why Passive?
My new Schecter Tempest guitar came with EMG 85 and 60 pickups installed. The guitar was manufactured in 2007 as a “ Hellraiser” model. Put EMGs in a guitar and give it an evil name and it becomes a metal guitar. I gave the EMG’s a couple of weeks playing and practicing and while they sound very good there was something missing. The EMG tone is very consistent and even throughout the range of the volume control and the tone controls but lacks a certain “feel” compared to a passive pickup. Some of the differences have to do with the way a passive pu interacts with the volume controls, tone controls and when both pickups are selected, the way you can blend the two PU for different sounds. This is due to the fact that passive PU are high impedance devices and form a LCR tuned circuit that creates tone changes as the controls are manipulated. I like the tones from passive circuits and I don’t hear the same tones when using the EMGs…so they had to go.
Mechanical changes
In order to convert the guitar to passive pickups all the internal controls must be changed to the higher impedance values required. The only thing I did not change was the selector toggle and the output jack. The output jack has a ring connection used to switch the internal battery when a plug is inserted into the jack (just like an effects pedal). I want to use the internal battery later.
I removed the EMGs and the internal controls. DSCN0207[1]One thing I noticed about the stock wiring is that the tone controls were wired different for each pickup. The bridge used the standard “modern” wiring while the neck used “50’s style” with the tone control connected to the volume pot wiper. DSCN0211[1]
Here is the empty control cavity.
DSCN0213[1] There is a very thin layer of conductive paint applied that scratches very easily. I don’t know how effective the paint is but two coats would provide better protection against scratches.
After removing the pickups controls and wiring the holes in the body had to be enlarged to accommodate the 3/8 bushing of the new CTS pots. The stock Asian pots have a smaller bushing as you can see. DSCN0210[1] I used a step drill bit to enlarge the holes after taping the top to prevent chipping. In this case I used a hand drill to slowly enlarge the holes with good results.
I knew I was rewiring the entire control cavity so I decided early on that all wiring would be shielded with a single point (star) ground and no huge solder lumps on the back of the pots. I used a small piece of vector board to terminate all the connections and provide a place to install a buffer circuit later. Here are the pots all wired up ready to go.
The tone pots have only two connections required but the board end of the connection has a grounded shield. Here is the finished wiring.
Circuit design
I have never been a fan of typical tone control circuits and sometimes I disconnect them completely. I wanted to have something a bit more useful in this guitar. I had been reading about the Bass contour control in Reverend guitars and the PTB controls on G&L guitars. The bass contour sounded like a good option to coil splitting to get a leaner tone from a humbucker. After some experimenting I decided on the following circuit:
• Volume control (500k) for each pickup
• A treble roll off control for the bridge pickup with a 500k no load pot
• A bass contour control for the neck pickup (500k reverse audio)

The bridge tone control uses a .01ufd cap and with the no load pot I can completely take it out of the circuit. There is a slight but noticeable difference with the tone pot on 10. The neck pickup bass contour control uses a .002ufd capacitor and a reverse taper audio pot. The reverse taper is required to get the control to work properly.
I did have a fortuitous mistake when wiring the circuit by connecting the bridge pickup out of phase with the neck. Nothing new there as I get the standard out of phase tone in the selector middle position. However when I used the bass contour control in the selector middle position with the pickups out of phase I found a whole new range of tones. Effectively this gives a half-out of phase tone with lower end added to the thin out of phase tone. This not a new idea having been used by Bill Lawrence in the Gibson L6-S and more recently in the Peavey Omniac, but both of those are static settings where as the bass contour control is variable from full out of phase to something in between.
I haven’t decided to keep the out of phase tone but the bass contour is very useful in cleaning up the neck pickup low end while retaining the noise cancellation of the humbucker.

More to do

I will post more as this project progresses. Two things that are still outstanding are the string grounding and the buffer circuit. Most EMG equipped guitars do not have string grounding as the pickups are very quiet without it. The passive high impedance Duncan’s require a string ground to control noise especially with high gain or overdrive pedals. The buffer will be added to keep the passive circuitry from interacting with various cables and pedal inputs and maintain consistent tone and control response no matter what it’s plugged into.

Parts list
Seymour Duncan SH-15 Alternative 8
Seymour Duncan SH-6n Seymourizer
• 2 ea. CTS 500k volume pot
• 1 ea. CTS 500k no load tone pot
• 1 ea. CTS 500k RA bass contour pot
• .01 ufd tone capacitor
• .002 ufd bass capacitor
• Two conductor shielded cable
• Small Vectorboard or perf board material